In photolithography systems, projection optics form an image of an illuminated object, such as a reticle, on a substrate, such as a semiconductor wafer or flat panel display substrate, that is coated with a photoresist. Illuminating the photoresist to expose the substrate, then etching the exposed substrate causes a pattern to form on the substrate that mimics the image with a resolution determined by the optical system and the size of the features on the reticle. At present, the desired feature size for applications in the semiconductor and flat panel display industries is under three microns and continues to shrink.
Even minute changes in the relative positions and attitudes of the reticle, lens, and substrate can adversely affect the image quality. In mechanically stable systems, heating and cooling of the lens by the illumination source may be the largest source of error. During exposure, the lens absorbs some of the light from the illumination source, causing changes in both the refractive indices and shapes of the optical elements in the lens, which, in turn, shift the focal plane away from the surface of the substrate. At the high doses used in some processes, the focal plane can shift significantly during a single exposure, blurring the image recorded in the photoresist.
Prior systems compensate for focal plane shift using more stable structures, more frequent calibration, and more interferometric measurements. For example, metal blocks can be used to compensate, actively and passively, for expansion or contraction in the lens and lens supports. As the blocks change temperature, either because they absorb incident light or are actively heated or cooled, they expand and contract to offset simultaneous expansion and contraction of the lens. Thermal compensation can also be used with models of focal plane shift derived from calibration data or with interferometric measurements of the relative positions of the substrate and the lens. Unfortunately, thermal compensation is slow, and interferometric measurement requires bulky equipment and optical access to the substrate or the substrate stage. In addition, factors besides thermal gradients, such as changes in atmospheric pressure, can cause focus changes without changing field curvature and astigmatism.
Alternatively, changes in focus can be measured and corrected (or compensated) using alignment marks, or fiducial marks, to determine the relative locations of the focal plane and the substrate. U.S. Pat. No. 5,991,004 to Wallace et al., incorporated herein by reference in its entirety, describes using sensors and gratings to measure defocus in photolithographic systems due to thermal effects. The Wallace system images a first grating in an object plane of the lens onto a second grating in an image plane of the lens. A relay lens images the second grating onto a detector, which records a Moiré pattern with a fringe spacing determined by the periods of the first and second gratings.
In the Wallace system, tilting the second grating about its center with respect to the image plane moves the edges of the second grating out of the lens's depth of field. As a result, if the lens focus is at the image plane, then the Moiré pattern appears only at the middle of the detector because the edges of the second grating are out of focus. As the focus shifts towards or away from the image plane, the Moiré pattern shifts left or right, depending on which way the second grating is tilted. Although this is a simple way of measuring defocus in real time, monitoring focus change in this way at only one radius in the field does not ensure determination of higher-order aberrations, such as astigmatism and field curvature. In addition, the gratings are not on the reticle or the substrate, so they must be registered precisely to the reticle and the substrate to track defocus accurately.
U.S. Pat. No. 7,301,646 to Wegmann et al., incorporated herein by reference in its entirety, extends the use of periodic structures and substructures to measure astigmatism and Petzval surface position. Like the Wallace system, the Wegmann system includes gratings at the object and image planes, respectively, of a projection lens. In the Wegmann system, however, there are four pairs of gratings, each of which is oriented in a different direction (e.g., x, y, and ±45°). Measuring the phase of the wavefront transmitted through a particular pair of gratings yields a wavefront whose phase depends, in part, on aberrations due to thermal effects on the lens.
Unlike Wallace, Wegmann's gratings are printed on the reticle and imaged through the plane of the substrate. This complicates the design because either or both the production pattern to be printed and the substrate interfere with the optical and mechanical design. In addition, the cameras used to image the gratings must remain stable between and during measurements. If the camera drifts between measurements, then it will record an image that is shifted due to misalignment of the camera (plus any misalignment of the object and image planes).
U.S. Pat. Nos. 6,320,644 and 6,483,572, both to Simpson et al. and incorporated herein by reference in their entireties, describe alignment methods that eliminate the effects of camera misalignment. In the Simpson patents, alignment involves projecting fiducial marks on or embedded in the reticle through a lens system. The images of the fiducial marks on a reticle are detected with a camera mounted to a metrology plate that is coupled to the lens system. Because the camera is mechanically coupled to the lens, it is always aligned properly to the lens, provided that the mechanical coupling is stable. As a result, aligning the camera to the reticle also aligns the lens to the reticle. The fiducial marks, camera, and/or lens must be aligned to the substrate, however, for the substrate to be aligned to the reticle. In addition, fixing the camera (and the metrology plate) to the lens limits the flexibility of the system by increasing the bulk and mass of the lens assembly.